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Much data can also be obtained regarding the mineral composition of iron slag
by examining it with a microscope.
Smelting slag is composed primarily of fayalite (Fe2SiO4) and silicate glass
substances. When iron sand high in titanium (i.e., akome) is used, the slag will
contain crystals of mineral titanium, which is composed of ulvospinel (Fe2TiO4)
and ilmenite (FeTiO3). Based on the state of these crystalline structures, it
is possible to make estimates regarding the temperature and rate of cooling at
which they were produced.
In the case of okaji slag (also called refining smithing slag), the volume of
Ti inclusions are extremely small, and there is almost no crystallization of
mineral titanium, even with slag where iron sand was used. Instead, when the
presence of iron in the makeup is high, at more than 50%, the volume of manufacturing
slag is also lower, and the iron oxide called wuestite (FeO) in the mineral structure
is crystallized. Dendritic crystals of wuestite will appear in small amounts
even in kera-oshi smelting slag. It forms an intermediate structure in zuku-oshislag and okaji slag.
Wuestite is present in large amounts in kokaji slag (also called forging smithing
slag), producing another level. Also, small scraps of iron (oxidized scale from
the iron) produced during forging accompany the slag.
Shown for reference below are the typical mineral structure of iron slag and
a chemical analysis of smithing slag.
Microscope photos of the mineral structure of typical iron slag
(all photos are
at the same magnification)
| Figure: Chemical
composition of typical smithing slag (mass %)
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| NOTES: (1) Kanayago tatara;
(2) Yasukuni tatara; (3) Mt. Tsugo tatara
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We can glimpse of one aspect of ancient iron manufacturing through this type
of research on iron slag. As a result, we know that the manufacture of iron
using prospered on the San'yo side of the region in ancient times. We know
also that refining and smithing (okaji) took place during the Yayoi Period
(B.C.E. 330 to 330 C.E.), but that iron manufacturing did not. It is also clear
that iron manufacturing began to take place from around the 6th century C.E.
However, this is all based on conjectures made about ancient techniques that
rely on data about iron slag obtained through the use of post-Meiji Period tatara technology.
If the ancient technologies differed significantly from those in use since the
Meiji Period, then our presumptions will have been mistaken. For
example, we don't know about the iron slag produced when smelting in a furnace
where both the temperature and reducibility are low. It might have the intermediate
characteristics of kera-oshi and smithing slag, and someone might decide that
it is smithing slag. In the future, it will be necessary to conduct research
on iron slag produced through such basic iron-making methods.
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